Gas, coal and oil burning furnaces are very prevalent in our society and have many industrial, commercial and residential applications. For example, in the refining of oil to produce gasoline and other petroleum products, crude oil must be heated in large outdoor furnaces as a part of the refining or "cracking" process. It is common in oil refineries for as many as 25 to 50 furnaces to be in constant 24-hour operation. These furnaces consume massive amounts of oil and natural gas. In fact, the annual fuel costs incurred in operating each one of these furnaces often exceeds $200,000, depending upon the size of the furnace. Therefore, small improvements in the efficiency of the operation of the furnace can yield large savings in fuel cost.
The efficiency of the furnaces' operation can be determined by measuring the amount of fuel or air which is not consumed in the combustion process. Perfect combustion, the most efficient form of combustion, occurs when the fuel and air are completely consumed, and only the gaseous by-products remain. Thus, under perfect combustion conditions, there is a 0% excess oxygen in the combustion by-products. Naturally, the more efficient the combustion, the less fuel which is consumed in heating the crude oil during the cracking process. In order to avoid inefficient excess oxygen conditions during the furnaces' combustion, it is essential that the amount of air entering the furnace, or in other words the "draft", not exceed the amount of draft needed for perfect combustion. Otherwise, the furnace will be operating inefficiently.
In contrast to the problem of excess oxygen caused by excessive draft conditions, perfect combustion can also be prevented by the presence of excess fuel, or in other words insufficient draft. Thus, the draft conditions in a furnace are extremely important in maintaining its efficient operation and the conservation of fuel. Furthermore, the maintenance of even, uniform draft in the furnace, under both mild and extreme changes in furnace operating conditions, is critical to combustion efficiency.
Heretofore, improper draft control has been the major contributing factor in inefficient furnace operation. Changes in the draft volume or velocity may be caused by many external factors, such as the amount of crude oil fed into the furnace or changes in the ambient condition surrounding the furnace. For example, an increase in the furnace's fuel consumption, necessitated by an increase in the crude feed rate, can produce oxygen-deficient combustion, resulting in dark smoke and increased pollutant emissions. Furthermore, a strong wind blowing across the top of the stack of flue of the furnace will create a low-pressure area at that point, increasing the amount of draft and resulting in excess oxygen conditions and inefficient combustion. Also, a decrease in temperature of the surrounding air will increase its density, again resulting in excess oxygen conditions.
It is common to compensate for these changes in draft intensity by providing a furnace with a damper plate in the flue to control the velocity of the draft. However, these flue dampers must be manually operated. Therefore, their position must be adjusted with each change in furnace operating conditions, whether it be a change in fuel rate or ambient conditions. This method of draft control increases the labor costs associated with furnace operation. Furthermore, since these flue dampers are exposed to the hot flue gases, their bearings deteriorate very rapidly, causing the damper to stick and become inoperable.
Another prior approach was to mount a damper plate in a channel in communication with the flue but not directly exposed to the hot flue gases. Although the damper plates of the prior art were somewhat efficient in controlling draft intensity, they have not been sufficiently sensitive to changes in draft conditions, especially extreme changes in operating conditions. For example, prior art plates were not able to permit the flow of ambient air into the flue until the pressure differential across the plate was sufficient to overcome its own weight. Therefore, the response of prior damper plates was slow, permitting inefficient draft conditions to persist. On the other hand, a plate whose weight was only barely sufficient to maintain it in a closed position flapped widely and uncontrollably in response to extreme changes in draft conditions. For example, a strong wind blowing across the top of the flue caused prior damper plates to open and close very widely and rapidly, preventing the even, uniform control of the draft.
A further substantial disadvantage associated with the prior damper plates is that they normally operate in a closed position, totally preventing outside, auxiliary draft air from entering the flue. Under these conditions, if the velocity of the draft should be increased (for example, when the fuel consumption of the furnace is increased), the damper plate is unable to permit more air to be drawn in at the base of the furnace. Thus, insufficient oxygen conditions exist, resulting in inefficient furnace operation.